Heavy rain that pelted the densifying city of Denver Sunday night dumped 1.76 inches of water in 26 minutes — exceeding norms in a flood-prone region, state and federal meteorologists concluded Monday as they reviewed measurements.
Intense bursts of rain fits patterns linked to climate warming — because warmer air can hold more water, packing more of a punch when atmospheric currents create storms.
“You have more dry periods, but, then, when it does rain, the rain is more intense. That’s the general picture of what a warming climate does to the water cycle,” state climatologist Russ Schumacher said at Colorado State University.
Flooding increasingly plagues concrete-heavy cities, including Denver, where urban drainage officials advocate a shift to “green infrastructure” that could protect residents. Denver developers for decades have poured more and more concrete, covering natural terrain, creating increased potential for water to collect on surfaces.
The weather monitoring stations run by Denver’s Mile High Flood District east of downtown near the zoo measured Sunday night surges at 1.42 inches in 20 minutes, 1.3 inches in 15 minutes, and 1.76 inches in 26 minutes, National Weather Service meteorologists said.
“Certainly notable, very intense rain, no question about it,” Schumacher said after reviewing those numbers.
Climate scientists long have anticipated heavier rain. Every 1 degree Celsius increase in temperature (1.8 degrees Fahrenheit) means an increase by 7% in how much water vapor the air can hold. When atmospheric currents carry clouds over a region, more rain falls. A trend toward “the heaviest rains getting heavier” has been documented over the Northeast and Midwest, Schumacher said.
In the Southwest, rainfall overall has decreased during a long-term drought since 2000, though Colorado faced heavy rain such as the storms in 2013 that led to historically extreme flooding.
“We have a history of very bad floods here along the Front Range. Even if we think of the types of storms we know happened in the past, it would be a big challenge to have infrastructure robust enough to withstand the rain. But the trend will probably appear here in the future with short-term intense rains getting more intense. That further increases the infrastructure challenges.”
How much water collects and floods an area depends on where the rain falls and the type of terrain, National Weather Service meteorologist Dave Barjenbruch said, calling the rainfall Sunday night in northeast Denver a “pretty significant” storm.
“Say you are in a forest. There’s a lot of shelter, and water easily infiltrates into the soil,” Barjenbruch said. A cornfield, or open space, would be able to soak up sudden heavy rain.
But in dense-packed cities the combinations of concrete, asphalt and hard-packed dry soil “tend to repel water,” he said, comparing Denver with wildfire burn scars in western Colorado where scorched soil renders landscapes impervious to falling rainwater.
“You’re going to get more runoff.”
Denver’s Mile High Flood district research and development director Holly Piza on Monday acknowledged challenges and pointed to benefits of natural terrain.
“Paving green space results in increased stormwater runoff and flooding,” Piza said. “The Mile High Flood District recommends good planning and solutions that use green infrastructure, such as routing paved areas onto pervious areas and keeping streams open — not piped.”
